Grapefruit juice increases the oral availability of certain drugs, such as felodipine, by limiting first pass metabolism in the intestinal wall. Some evidence suggests that 6'7 dihydroxybergamottin (DHB) is the principle ingredient in grapefruit juice responsible for this effect. Seville orange juice contains DHB in concentrations comparable to that observed in grapefruit. We will test the hypothesis that DHB is the active ingredient in grapefruit juice by comparing the effects of Seville orange and grapefruit juices on the oral availability of felodipine in healthy volunteers.